Current emission control regulations necessitate the use of exhaust aftertreatment systems in order to reduce the concentration of combustion byproducts and/or products of incomplete combustion.
Spark ignition (i.e., gasoline) engines conventionally use three-way catalytic converters to satisfy emissions regulations. Compression ignition (i.e., diesel) engines, however, are typically equipped with two-way catalytic converters (also referred to as Diesel Oxidation Catalyst—DOC), which may not efficiently reduce nitrogen oxides (NOx). Accordingly, diesel engines may include a reductant-based selective catalytic reduction (SCR) device in order to seek reduction in NOx, often the most abundant and polluting component in exhaust gases. In addition, diesel engines may also include diesel particulate filters (DPF) for particulate matter control.
Urea-based SCR catalysts use gaseous ammonia as the active NOx reducing agent. Typically, an aqueous solution of urea, also known as carbamide ((NH2)2CO), is carried on board of the vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst where it decomposes into gaseous ammonia (NH3) and is stored in the catalyst. The NOx contained in the engine exhaust gas entering the catalyst then reacts with the stored ammonia, which produces nitrogen and water. It is worth noting that to attain the conversion efficiency required to achieve the emission limits, the SCR-catalyst must be at least at a predetermined temperature known as light off temperature.
In an engine's exhaust system, the SCR catalyst is conventionally placed downstream of other exhaust after-treatment means, namely the catalytic converter and possibly a particulate filter.
One of the challenges in such exhaust line with serially mounted exhaust aftertreatment devices is its thermal management. In the above case of an exhaust line comprising, in series, an oxidation catalyst, a particulate filter and a SCR catalyst, the catalytic elements should reach their respective light off temperature as early as possible.
But the SCR catalyst is usually the last treatment component in the exhaust line and the most remote from the engine exhaust valves. In addition, each component in the exhaust line has a thermal inertia, whereby the temperature increase of components situated further away from the engine is delayed as compared to those near the engine. Thermal losses in the exhaust pipe itself are also to be taken into account. FIG. 1 illustrates a typical temperature trace, in a diesel engine, for each of the diesel oxidation catalyst (DOC), the diesel particulate filter (DPF) and the SCR catalyst.
As can be seen, the temperature of the SCR catalyst is heavily dependent on the thermal behavior of the other components in the exhaust line. In order to heat-up the catalysts as soon as possible, the engine is normally operated in a heat-up mode by acting on engine control parameters to heat-up the temperature of engine-out gases or by injecting fuel in the exhaust line that is burned in the DOC. Such heat-up strategy is e.g. described in US 2009/0217645.
The heat-up mode may be triggered from engine start up. Conventionally the thermal management is then carried out by monitoring the current temperature of the SCR catalyst; and the heat-up mode is stopped as soon as the target temperature in the catalyst is reached, e.g. the light off temperature.
Unfortunately, under such control conditions the DOC gets very hot, while at the same time the SCR is still relatively cool due to thermal inertia of the components in the exhaust line. As can also be understood from FIG. 1, when the heat-up mode is stopped at, e.g., a light off temperature TLO of 250° C. measured in the SCR-catalyst, the thermal inertia in the system leads to an overshoot in the SCR temperature TSCR above 300° C. Such high temperatures are not required for NOx conversion and thus reflect a waste of fuel to heat-up the exhaust line.